1. Field of the Invention
The present invention relates to an image forming apparatus for forming images on sheets, and an information terminal for controlling the image forming apparatus, and more particularly to a technique for accurately detecting the remaining sheet amount of a sheet feed tray that feeds sheets on which images are to be formed by the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus that forms images on sheets, the remaining sheet amount of a sheet feed tray is detected by mounting one of various types of sheet detection sensors for detecting the remaining sheet amount on the sheet feed tray. When a change in the remaining sheet amount is detected by the sheet detection sensor, the image forming apparatus updates the remaining sheet amount of the sheet feed tray. More specifically, the image forming apparatus does not detect the remaining sheets in the sheet feed tray on a sheet basis, but stores values of the remaining sheet amount as respective detectable levels depending on the accuracy of the sheet detection sensor, and hence the remaining sheet amount has a predetermined range. Assuming that the sheet detection sensor detects the remaining sheet amount in four levels, the remaining sheet amounts to be detected are “0”, “⅓of the maximum value of the remaining sheet amount of the sheet feed tray”, “⅔of the maximum value of the remaining sheet amount of the sheet feed tray”, and “the maximum value ( 3/3) of the remaining sheet amount of the sheet feed tray”.
In general, a user using an image forming apparatus compares the number of sheets to be used in a print job the user is about to execute and a remaining sheet amount value detected by a sheet detection sensor, and considers whether or not there is a sufficient remaining sheet amount for executing the print job. If the user determines that the remaining sheet amount is insufficient, the user is required to replenish the sheet feed tray to make up for the insufficiency of sheets. At this time, whether or not an accurate remaining sheet amount value can be notified to the user depends on the detection accuracy of the sheet detection sensor.
For users who rarely perform large-volume printing, the necessity for acquiring an accurate remaining sheet amount value may be low. However, users under a POD (print on demand) environment often execute a print job for printing a large number of pages. The user schedules the print job, and performs printing in such a manner that the image forming apparatus is not stopped e.g. due to a sheet run-out error. When using the image forming apparatus in such a manner, it is necessary for the user to know what kind of sheets are required to be set in a sheet feed tray in what amount, and hence the image forming apparatus is needed to acquire a remaining sheet amount value which is as accurate as possible and provide the same to the user.
As a technique meeting such a need, there has been proposed a technique which corrects a remaining sheet amount value by using a counter for counting the number of sheets held in the image forming apparatus and a sheet detection sensor provided for a sheet feed tray (see Japanese Patent Laid-Open Publication No. H11-1051). In this technique, when the corrected remaining sheet amount value becomes lower than a detection value to be detected by the sheet detection sensor (e.g. when the corrected remaining sheet amount value is not larger than 0), the correction of the remaining sheet amount value is not performed.
In the above-described conventional technique, the correction of the remaining sheet amount is performed only on a single sheet feed tray, and hence even after the remaining sheet amount value of the single sheet feed tray is finally determined, sheet feed from the single sheet feed tray continues to be executed. On the other hand, an image forming apparatus is widely used which includes a plurality of sheet feed trays and is capable of feeding a large number of sheets. The user who uses such an image forming apparatus under the POD environment sets sheets of the same type and the same size in a plurality of sheet feed trays. Further, an automatic sheet feed tray-changing function is sometimes used which, when one sheet feed tray becomes empty (one sheet feed tray runs out of sheets), enables sheets to be automatically fed from another sheet feed tray in which sheets of the same type and the same size are set.
If such a method of using the image forming apparatus is applied to the above-described conventional technique, after the remaining sheet amount value of one sheet feed tray is accurately detected, sheets continue to be fed from the same sheet feed tray, and when the remaining sheet amount value of the sheet feed tray becomes equal to 0, the sheet feed tray is automatically changed to a next sheet feed tray so as to feed sheets from the next sheet feed tray.
However, this brings about the problem that when a plurality of sheet feed trays are set to the same type and the same size, it is impossible to sum up the remaining sheet amounts of all the sheet feed trays and check the total sheet amount. The same problem also occurs in an image forming apparatus which is capable of grouping sheet feed trays and setting sheets of the same type and the same size in a group of a plurality of sheet feed trays at a time.